1. Field of the Invention
The present invention relates to an improved gypsum board for use in building construction and to a process for its manufacture; and more particularly, to a gypsum board having a gypsum matrix with glass fibers having a tacky bond layer thereby providing superior gypsum board flexure strength and nail pullout resistance.
2. Description of the Prior Art
Gypsum wallboard and gypsum panels are traditionally manufactured by a continuous process. The conventional process for manufacturing gypsum wallboard includes premixing of dry ingredients of the core composition, which can include calcium sulphate hemihydrate (CaSO4.½H2O, also known as calcined gypsum, stucco, and plaster of Paris), accelerator, starch, glass fiber and others. The premix of dry ingredients is then mixed with “wet” portion of the core composition in a pin mixer. The “wet” portion can include water, foaming agent, paper pulp, fluidity-increasing agent, and other conventional additives. Various additives, e.g. cellulose and glass fibers, are often added to the slurry to strengthen the gypsum core. Starch is frequently added to the slurry in order to improve the adhesion between the gypsum core and the facing.
The resulting gypsum slurry is continuously deposited to form a gypsum wallboard core between two continuously supplied moving sheets of cover paper. The two cover sheets are typically a pre-folded face paper and a backing paper. As the gypsum slurry is deposited onto the face paper, the backing paper is laid over the gypsum slurry and bonded to the pre-folded edges of the face paper with a suitable adhesive. The enclosed gypsum core slurry is then sized for thickness through forming plates or roller bars and allowed to set between two cover sheets, thereby forming a board. The setting process is a rehydration reaction that transforms calcium sulfate hemihydrate to calcium sulfate dihydrate, shown as follows.CaSO4.½H2O+ 11/2H2O→CaSO4.2H2O+Heat
Once the gypsum core has set sufficiently, the continuously produced board is cut into desired lengths and vertically stacked. After the cutting and stacking step, the gypsum boards are fed into drying ovens or kilns to evaporate the excess water. Inside the drying ovens, the boards are blown with hot drying air. After the dried gypsum boards are removed from the ovens, the ends of the boards are trimmed off and the boards are cut to desired sizes. The boards are commonly sold to the building industry in the form of sheets. These sheets are usually 4 feet wide, 8 to 12 feet long and 0.5 to 1 inches thick, the width and length dimensions defining the two faces of the board.
Wallboard formed of a gypsum core sandwiched between facing layers is used in the construction of virtually every modern building. In its various forms, the gypsum board is used as an interior or exterior surface for walls, ceilings and the like. The gypsum board is relatively easy and inexpensive to install, finish, and maintain, and depending on the composition of the gypsum matrix, may be relatively fire resistant. A number of patents discuss various reinforcement fibers with polymeric coatings within gypsum and other hydrated matrices.
U.S. Pat. No. 4,241,136 to Dereser et al. (hereinafter the '136 patent) discloses a process and composition for treating glass fibers for use in reinforcement of cementitious materials. The fibers are first sized with a cationic fiber forming organic polymer and then with a second coating containing an anionic film-forming organic polymer. The resulting fibers are said to have good wetting and dispersibility characteristics. The '136 patent suggests that the high surface charge density of asbestos fibers, in combination with a high specific surface area, permit them to flocculate cement mixed therewith, thereby providing a substantial degree of reinforcement to structural articles. However, replacement of asbestos fibers with glass is said not to have the expected benefit, in that the glass fibers tend to adhere together and thereby inhibit the removal of water during mat or board production. In addition, the much lower specific surface area of glass fibers results in poor retention of either cement or water thereon, in comparison with asbestos. The glass fibers do not have similar surface charges and the '136 patent sizing process is ineffective in bonding exclusively glass fibers without asbestos. Furthermore, the sizing utilized in the '136 patent is not a polymeric coating which is tacky.
U.S. Pat. No. 4,935,301 to Rerup et al. relates to a cement composite containing glass fibers encapsulated with a polymeric coating which is formed from an organic solution of an interpolymer complex of an anionic polymer and a cationic polymer. The fiber reinforcement is said to impart to the composite improved high apparent toughness, ductility, and flexural and tensile strengths, along with improved resistance to embrittlement and strength loss with age. The fibers are disposed in bundles, which are encapsulated with an elastomeric material. The encapsulant wraps the bundles of fibers but does not coat the individual fibers, nor does the coating impregnate the bundle or fill the voids between the individual fibers. The fibers are disposed in any cementitious matrix, including Portland cement, concrete, mortar, gypsum, and hydrous calcium silicate. The elastomeric coating is applied in an organic solvent and is not applied in a water-based system. The composite formed is primarily a cement composite, even though gypsum composite is casually mentioned in the patent. The anionic and cationic polymers are related to bond creation with the fiber prior to incorporation in the matrix and do not result in a tacky bond.
U.S. Pat. No. 5,100,474 to Hawkins (hereinafter the '474 patent) discloses a glass fiber reinforced composition of a settable mix of plaster, a water-based phenol formaldehyde resin, an acid hardener, and reinforcing glass fibers. The invention is said to advantageously have a lower resin content compared to glass fiber plastic products produced by known laying up techniques resulting from the incorporation of plaster. The '474 patent further discloses three methods of producing flat sheet and molded form products: (i) pre-mixing of the constituents, followed by pouring or injection into open or closed molds; (ii) hand lamination; and (iii) hand spray lamination. The formaldehyde resin does not form a tacky bond layer between the plaster matrix and glass fiber.
U.S. Pat. No. 5,786,080 to Andersen et al. discloses compositions and methods for the deposition of ettringite (3CaO—Al2O3.3Ca(SO4).30-32H2O) onto the surfaces of fibers, aggregates, or other fillers. The ettringite is produced in situ within an aqueous suspension while in proximity of the fibers, aggregates, or fillers, to form a mineralized composite material comprising ettringite coated fibers, aggregates or other fillers. The ettringite-coated materials can be added to hydraulically settable materials to improve the chemical and mechanical bond between the fibers or other substrate within the resulting hardened hydraulically settable materials, particularly cementitious or concrete material. The presence of the coated fiber materials is said generally to increase the toughness, flexibility, tensile strength, and flexural strength of the composite and articles made therefrom. It is indicated that the ability of fibers to modify the mechanical properties of a composite is dependent on the strength of the bonding between the fibers and the matrix material. The ettringite process is said to increase the roughness of the coated fibers, thereby enhancing the mechanical interlocking with the matrix over that achieved with relatively smooth glass fibers. The ettringite composition is an inorganic coating and not a tacky polymeric coating.
U.S. Pat. No. 5,879,825 to Burke et al. discloses a gypsum wallboard and method of making same. The board is made with a core of calcium sulfate hemihydrate (stucco), water, and a strengthening agent from a slurry. The slurry contains paper pulp fiber reinforcement. The strengthening agent is an acrylic polymer composition having a glass transition temperature of about 15° C. or greater, and preferably has good divalent ion stability so that it is suitable in a medium of calcium ions. The method provides wallboard having increased core strength, paper-to-core bonding, and strength-to-weight ratio. The gypsum board is not reinforced with glass fibers and the acrylic polymer does not create a tacky bond between the glass fiber and the gypsum matrix.
U.S. Pat. No. 6,171,388 to Jobbins discloses a lightweight gypsum composition. The composition comprises (a) gypsum (CaSO4.2H2O); (b) one or more naturally occurring or synthetic latex polymers including ethylenically unsaturated monomers selected from (meth)acrylic based acids and esters, acrylonitrile, styrene, divinylbenzene, vinyl esters, acrylamide, methacrylamide, vinylidene chloride, butadiene and vinyl chloride and mixtures thereof; and (c) one or more nonionic surfactants; wherein the gypsum composition has a density less than 0.64 g/cm3. The gypsum board is made from a slurry that has low density due to foaming and has no glass fibers incorporated within the gypsum matrix. The bond is therefore between gypsum-hydrated crystals using the latex binder, not between a glass fiber and gypsum matrix. Further the bond is not tacky.
U.S. Pat. No. 6,254,817 to Cooper et al. discloses a composite fabric for use in reinforcing cementitious boards and similar prefabricated building wall panels. The fabric is embedded in the cement matrix and closely adjacent to the surface (e.g. within 1/16″) of the panels or boards. The fabric preferably comprises a mesh of continuously coated, high elastic modulus strands, which are preferably bundled glass fibers encapsulated by alkali and water resistant thermoplastic material. The disposition of the fabric near the surface advantageously optimizes reinforcement. Preferred coating materials include polypropylene, polyethylene, copolymers of polybutylene and propylene, ethylene propylene rubber, thermoplastic polyolefin rubber, polyvinylidene chloride, and ethylene-propylene diene monomer. The cement board is not a gypsum board and the glass fiber is coated with the thermoplastic coating prior to insertion in cement board. The coating of these polymeric materials does not result in a tacky bond layer.
U.S. Pat. No. 6,294,253 to Smith, Jr., discloses a sized, staple fiber product useful in the manufacture of gypsum board. The fiber surface is coated with an aqueous chemical size composition containing a high level of surfactant such as a poly (Oxy-1,2-ethanediyl), alpha(2-(bis(2-Aminoethyl)Methyl-ammonio)Ethyl)-omega-Hydroxy-, N,N′-Di(C14-18 and C 16-18 unsaturated) Acyl Derivs., Me Sulfate (Salts) and optionally, a polymer film former such as polyvinyl alcohol and a biocide. The sized fibers may ultimately be incorporated as reinforcements in the gypsum core of a construction board. Preferred fibers are 5-23 μm in diameter and chopped to less than 1.5 inches long. The sizing provides a thin polymeric coating that is not tacky.
U.S. Pat. No. 6,524,679 to Hauber et al. discloses a glass reinforced gypsum board. A multilayer gypsum board having face sheets comprising inorganic fiber, preferably randomly oriented glass fiber, which have been completely impregnated with a gypsum slurry so as to penetrate through the random interstices between the inorganic fibers and to thereby coat the board surfaces with gypsum slurry. The multilayer gypsum board may have a polymeric compound added to unset gypsum, the compound may comprise any of the following: polyacrylamide, polymethylacrylamide, polyvinyidene chloride (PVDC), polyamide, poly (hexamethylene adipamide), polyvinylchloride (PVC), polyethylene, cellulose acetate, polyisobutylene, polycarbonate, polypropylene, polystyrene, polychloroprene, styrene, butadiene, natural rubber, poly (2,6 dimethyl pentene oxide), poly (4-methyl-1-pentene) and polydimethyl siloxane. The multilayer gypsum board may comprise a first layer of a mixture of set gypsum having an outer surface and the polymeric compound additive entrained within the set gypsum and being impregnated in a thin sheet of randomly aligned inorganic fibers so as to essentially encase the core gypsum within two facing layers having a combination set gypsum and polymeric compound. A multilayer gypsum board is formed by incorporating glass fibers together with polymeric additives to bury the fibers within the top and bottom surfaces of the board. The edges may be reinforces and continued on to a portion of the top surface as shown in the figure. The inorganic glass fibers are individually incorporated on top and bottom of the cast gypsum but not within the cast gypsum board and the gypsum slurry enriched with polymers. The glass fibers and the polymer is not present within the entire gypsum matrix and does not form a tacky gypsum matrix fiber interface.
U.S. Pat. No. 6,525,116 to Sethuraman et al. discloses a gypsum composition with ionic styrene butadiene latex additive. The gypsum composition comprises functionalized styrene butadiene latex polymers crosslinked by a dimethacrylate having from 2 to 30 ethoxy units between methacrylate functionalities prepared by aqueous emulsion polymerization of a monomeric mixture comprising styrene and butadiene in the presence of a seed polymer. The functionalized styrene butadiene latex is added to gypsum slurry. There are no glass fibers incorporated in the gypsum slurry and the latex bond is provided between gypsum crystals in the form of uniform dispersion. The functionalized latex formed is crosslinked and is therefore not tacky.
U.S. Pat. No. 6,755,907 to Westerman et al. discloses a gypsum composition with styrene butadiene latex additive. Gypsum wallboard made lighter and less dense, without sacrificing strength, by adding to the gypsum slurry used in making the board a styrene butadiene polymer latex substantially stable against divalent ions in which the styrene butadiene polymer latex substantially stable against divalent ions in which the styrene butadiene polymer includes at least 0.25 wt. % of an ionic monomer. The latex polymer is added to gypsum slurry, which does not have glass fibers. The latex polymer does not form a tacky bond layer on glass fibers.
U.S. Patent Application U.S. 2003/0134079 to Bush et al. discloses a method and composition for coating mat and articles produced therewith. The coated glass mat comprises a glass mat substrate having non-woven glass fibers and a coating, which essentially uniformly penetrates the glass mat substrate to desired fractional thickness of the coated glass mat. Coating is preferably a coating blend comprised of water, latex binder, inorganic pigment, and inorganic binder. The coating imparts a tensile strength to the coated glass mat, which on average is at least 1.33 times greater than the tensile strength of the glass mat substrate without the coating. Moreover, a non-coated thickness of the coated glass mat is sufficiently thick for bonding purposes with, for example, a gypsum slurry or other core materials such as thermoplastic or thermosetting plastics. The coating has porosity, which provides the coated glass mat with a porosity sufficient to allow water vapor to escape from a gypsum slurry when heated. A glass mat with non-woven glass fibers is coated with water, latex binder, inorganic pigment, and inorganic binder using a larger wrap kiss coater. The binder penetrates 25% to 75% of the thickness of the mat, leaving a rough uncoated free surface. The mat may be placed on gypsum slurry, thereby reinforcing the gypsum board. The non-woven glass mat is bonded with water, latex binder, inorganic pigment, and inorganic binder and the binder is not water soluble or reversible. The glass fiber mat is placed on top of gypsum slurry forming the gypsum board and not incorporated within the gypsum board. The bond between the fiber and the gypsum matrix is not tacky.
U.S. Patent Application 2004/0082240 to Rodrigues (hereinafter the '240 patent application) is directed to a fiberglass nonwoven binder. It employs aqueous solution of a copolymer binder having a monomer or acid functionality and a monomer of hydroxyl or amine functionality applied to hot nonwoven fiberglass fibers and heat cured to form a fiberglass mat that is strongly bound, yet flexible. The '240 patent application discloses a number of acid functionality monomers and hydroxyl or amine functionality monomers. Specifically, it discloses acrylic acid [para 0011] a carboxylic acid monomer and triethanol amine, an amine functionality monomer that crosslinks without the need for external crosslinking agents [para 0020]. The monomer mixture polymerizes or crosslinks when it contacts the hot fiber surface, creating a bond at the contact points. This is strictly creation of bond between two fiberglass fibers, not between a glass fiber and gypsum matrix. The polymer is created during cure by crosslinking and does not pre-exist in the solution forming a tacky coating on glass fibers.
U.S. Patent Application 2004/0082241 to Rodrigues (hereinafter the '241 patent application) is directed to a Fiberglass nonwoven binder. It is a continuation in part of U.S. Patent Application 2004/0082240 (discussed herein above). The '241 patent application also relates to the use of polyamines as crosslinkers for a polymer binder. It employs an aqueous solution of a copolymer binder having a monomer or acid functionality and a monomer of hydroxyl or amine functionality applied to hot nonwoven fiberglass fibers and heat cured to form a fiberglass mat that is strongly bound, yet flexible. The '241 patent application discloses a number of acid functionality monomers and hydroxyl or amine functionality monomers and polyamine crosslinking agents. Specifically, the patent discloses acrylic acid [para 0013] a carboxylic acid monomer and triethanol amine, an amine functionality monomer that crosslinks without the need for external crosslinking agents [para 0023]. The monomer mixture polymerizes, or crosslinks, when it contacts the hot fiber surface creating a bond at the contact points. This is strictly creation of bond between two fiberglass fibers, not between a glass fiber and gypsum matrix. The polymer is created during cure by crosslinking and does not pre-exist in the solution forming a tacky coating on glass fibers.
Notwithstanding the advances in the field of gypsum boards and related articles, there remains a need in the art for a readily and inexpensively produced gypsum board having improved strength and flexure resistance with superior nail pull out resistance.